The invention relates to a circuit system for the generation of a control voltage dependent upon an alternating voltage.
This kind of circuit system has been described in the applicant's own patent applications Nos. P 28 30 784.8 and P 28 30 786.0 which have not been published at an earlier date. It is possible to employ these circuit systems as control voltage generators in a compander system of the kind as has for example been described in the US. Pat. No. 3,969,680. In the case of this compander circuit the input of the control voltage generator is supplied through another electronically controllable amplifier on compression with the alternating voltage output signal and on expansion with the alternating voltage input signal from an electronically controllable amplifier located in the useful signal path. The direct voltage output signal of the control voltage generator is fed both into the control input of the amplifier located in the useful signal path and also into the control input of the further amplifier. The control voltage generator acts in this context so that on exceeding of a threshold value by the alternating voltage fed into the input of the control voltage generator it generates a swiftly rising direct voltage which varies the amplification of the amplifier located in the useful signal path until the alternating voltage at the input of the control voltage generator has dropped back below the said threshold value.
A swift variation of the transfer constant of the amplifier located in the useful signal path is especially important when the level of the useful signal varies in jumps over extensive ranges. A jump in level from a small to a large value would otherwise give rise to a risk of the compressor output signal overshooting thereby producing saturation of the transmission channel. A jump in level in the reverse direction would result in the generation of audible noise signals at the output of the expander during the transition time following readjustment. For the achievement of a complementary behaviour of compressor and expander allowance should expendiently be made in the same manner both in respect of compressor and expander for the characteristics specific for the modes of operation of the expander and compressor.
The control voltage generator for controlling the transfer constant of the amplifiers located in the useful signal path must therefore furnish a direct control voltage which is swiftly adapted to the momentary level of the useful signal. Prerequisite for this is a short charge change time constant effective at large level jumps on the charging capacitor belonging to the control voltage generator. In the stationary state or in the case of slow variations in level the charge change time constant should however be large to make sure that the direct control voltage does not vary in rhythm with the useful signal frequency. Such behaviour would give rise to a large non-linear distortion factor.
The general term charge change is employed in this context because the above considerations have validity both for circuits in which the charging capacitor is charged with increasing alternating voltage amplitude and for circuits in which the charging capacitor is discharged. It follows that it is necessary to meet several contradictory requirements when fixing the charge change time constants of charging capacitors.
In the case of a slow reduction in the alternating voltage amplitude the charge of the charging capacitor will be changed slowly. This changing of the charge begins at the instant at which the momentary value of the alternating voltage amplitude falls below a given threshold value. The time constant of this slow change in charge is fixed so that the maximum permissible non-linear distortion factor is just reached at the lowest signal frequency.
The large charge change time constant interferes when the alternating voltage drops quickly, because e.g. in the case of the expander, the transfer constant is not reduced at a quick enough rate so that interference noises in the transmission channel which had been previously swamped or masked by powerful useful signals are now becoming audible. A short charge change time and thereby a small charge change time constant for the control voltage generator are required in case of a rapid reduction in the alternating voltage amplitude. So as to ensure undistorted reproduction of the last oscillation sequence of the powerful useful signal the changeover to a smaller time constant may however occur only after completion of the last oscillation sequence. In view of the fact that the longest period of oscillation occurs at the lowest signal frequency it is necessary to fix the time lag for the changeover from a large to a small time constant in accordance as a function of the lowest signal frequency. Whilst some non-swamped noise can be detected at the output from the expander when the useful signal amplitude is reduced to a smaller value prior to the lapsing of this time lag, it has to be borne in mind that the human ear takes a certain length of time to adapt itself from a loud to a low level noise impression. The non-masked noise will therefore not be noticed when the time lag for the changeover of the time constant is picked shorter than the physiological time span of adaptation of human hearing.
The statements made so far were concerned with operational cases in which a useful signal had been available for an extended period of time prior to the decrease in amplitude. There exist however also sound situations (lit:events) in the case of which the amplitude rises briefly and falls back immediately afterwards (e.g. sharp reports or bangs, striking of piano chords). The time taken by human hearing for a change from low level to loud sound plays a role in this case. The auditory sensitivity for low level sound is not removed by a very brief loud sound impression. This may lead to a situation in which interference noises become audible at the output from an expander controlled by way of large transfer constant within the above discussed time lag after the termination of the brief useful signal. According to the invention the time lag based on the lowest signal frequency is therefore cut down or controlled to be zero in the case of very brief useful signals. This cutting down of the time lag is permissible with regard to the maximum non-linear distortion factor, because very brief useful signals cannot possess any low frequency spectrum fractions (portions), which would have to be allowed for in fixing (rating) the time lag.